Lithium secondary batteries relying on the electrochemical reactions ofLi++e−→Liin the charge stage, andLi→Li++e−in the discharge stage have been vigorously studied and developed as a power source of portable electronic instruments or electric motor cars because they have a very high energy density in theory compared to other batteries and thus allow to manufacture small size, light weight batteries. The performance of portable electronic instruments is ever increasing in recent years and their power consumption is also increasing concominately therewith. For use as a power source of these instruments in particular, the power source is required to have a satisfactory discharge characteristics even under heavy loads. Following lithium batteries using an organic electrolyte solution referred to as “lithium ion battery”, studies on batteries using a lithium ion-conductive polymer functioning both as the organic electrolyte solution and also as a polymer separator of the prior art batteries are in progress. The lithium secondary battery using the lithium ion-conductive polymer is very attractive because of its remarkable advantages such as possibility of making the battery smaller and thinner in size and lighter in weight as well as leak free. Lithium secondary batteries of this type now available in the market use a porous matrix of ion-conductive polymer impregnate with or retaining an organic electrolyte solution (a solution of lithium salt in an aprotic polar solvent) therein. However, leakage of the organic electrolyte solution from the battery has not fully been prevented in various environments.
When metallic lithium is used as anode in the lithium secondary battery, one problem is how the growth of lithium dendrite on the anode is prevented for improving the charge-discharge cycle characteristics. Various studies have also been made in order to solve this problem. JP-A-6223877 and JP-A-8329983 are representative examples of such studies among them.
JP-A-6223877 proposes to provide a plurality of ion-conductive layers having different lithium salt concentrations between the cathode and the anode in order to prevent the growth of lithium dendrite on the anode. JP-A-8329983 proposes to provide a pair of electrolyte layers separately on the cathode and the anode, respectively and to give higher ion-conductivity to the layer on the anode than the other layer on the cathode in order to prevent internal short circuit from occurring due to the growth of lithium dendrite. The object of these proposals is to provide a lithium secondary battery having high reliability and excellent cycle characteristics by preventing the internal short circuit.
JP-A-2000/106212 proposes a lithium battery having improved battery performance upon high rate discharge. The battery includes three separate layers of an electrolyte gel in which at least one layer either on the cathode or anode is different in the composition of electrolyte gel from the electrolyte gel in the separator such that the concentration of lithium salt is always higher in the separator than in the cathode and/or anode. This proposal, however requires to form three electrolyte layers independently on the two electrodes and the separator formed by laminating the three layers together and necessarily results in increased number of interfaces between the electrolyte gel layers undesirably for decreasing the internal resistance within the battery and also increased number of steps for manufacturing the battery.
Although the performance of lithium secondary battery is advancing by the use of ion-conductive polymer in conjunction with the use of improved ion-conductive polymer on the anode, further improvements are still demanded in the various performance of lithium secondary battery such as charge-discharge cycle life, discharge characteristics at a high load and other properties. In addition, we have conceived to decrease the number of interfaces between various electrolyte layers by essentially dispensing with an independent layer of ion-conductive polymer in the separator and decrease the internal resistance of the battery correspondingly.